1. Field of the Invention
This invention relates to a sheet feeding apparatus (sheet feeder) used for an image-forming apparatus such as a copier or a laser beam printer. More particularly, the invention concerns a sheet feeder which can feed thick sheets such as envelopes one by one.
2. Related Background Art
U.S. Pat. No. 1,919,238 discloses a sheet feeder, which feeds sheets with the lowermost sheet in a sheet stack in contact with a feed roller.
Also, U.S. Pat. No. 3,963,339 discloses a sheet feeder, which feeds sheets with the lowermost sheet of a sheet stack in contact with a conveyor belt, and in which the sheet stack is urged against the conveyor belt by a lever.
Further, it is conceivable to put a weight on the sheet stack so as to increase the sheet-feeding force in cases where the lowermost sheet of sheet stack is fed by the feed roller.
However, since such a weight in such sheet feeder is constant, when a large number of sheets are stacked, the urging force is absorbed by an air cushioning effect between adjacent sheets, the effect being pronounced particularly between adjacent envelopes to be fed. In this case, the urging force is not sufficiently transmitted to the lowermost sheet, and defective sheet-feeding is liable to result from insufficient sheet-feeding force. Further, when sheets in the stack are reduced, double feeding is liable occur due to excessive urging force.